1. The Field of the Invention
The present invention relates to the field of bandgap voltage reference circuits. In particular, the present invention relates to circuits and methods for providing a temperature-stable bandgap voltage reference using differential pairs to provide a temperature-curvature compensating current.
2. The Prior State of the Art
The accuracy of circuits often depends on access to a stable Direct Current (DC) reference voltage. One class of circuits that generates DC reference voltages is called xe2x80x9cbandgap voltage reference circuits,xe2x80x9d or xe2x80x9cbandgap referencesxe2x80x9d for short. Bandgap references use the bandgap voltage of the underlying semiconductor material (often crystalline silicon) to generate an internal DC reference voltage that is based on the bandgap voltage.
Many bandgap references forward bias the base-emitter region of a bipolar transistor to form a voltage VBE across its base-emitter region. VBE is then used to generate the internal DC reference voltage. VBE does, however, have some first-order, second-order and higher order temperature dependencies. Many bandgap references substantially eliminate the first-order temperature dependency by adding a Proportional-To-Absolute-Temperature (PTAT) voltage to VBE.
One such bandgap voltage reference circuit is disclosed in U.S. Pat. No. 3,887,863 (hereinafter referred to as the ""863 patent), which issued Jun. 3, 1975 to A. P. Brokaw. The bandgap voltage reference circuit disclosed in the ""863 patent relies upon a bandgap cell that is commonly referred to as a xe2x80x9cBrokaw cellxe2x80x9d.
Referring to FIG. 1, a schematic representation of a standard Brokaw cell 100 is shown. The Brokaw cell 100 comprises a pair of bipolar transistors (Q1 and Q2) and a pair of resistors (R1 and R2). The area of the base-emitter regions in Q1 and Q2 are indicated by A and unity, respectively, wherein A is greater than unity.
Referring to FIG. 2, a schematic representation of a bandgap voltage reference circuit 200 is shown incorporating a Brokaw cell 100. In addition to the Brokaw cell 100, the bandgap voltage reference circuit 200 comprises an operational transresistance amplifier R, as well as a pair of resistors R3 and R4 that allow the reference output voltage (VOUT) to exceed the bandgap voltage.
During operation, a voltage of VBE develops across the base-emitter region of bipolar transistor Q2. In addition, a PTAT voltage (termed VPTAT) develops across resistor R2. The base-emitter voltage (VBE) of a bipolar junction transistor has a negative temperature coefficient generally between xe2x88x921.7 mV/degree C. and xe2x88x922 mV/degree C. In other words, if the operating temperature of a bipolar transistor was to increase by one degree Celsius, the base-emitter voltage would decrease by a voltage in the range of from 1.7 to 2 mV. In contrast, the PTAT voltage has a positive temperature coefficient. In other words, as the temperature increases, so does the PTAT voltage. By matching the temperature coefficient of VBE of Q2 to the temperature coefficient of VPTAT of R2, the first order temperature coefficient of VB can be made zero (or at least very close to zero) thereby significantly reducing temperature dependency.
Although the bandgap voltage reference circuit substantially eliminates first-order temperature dependencies in the output voltage, second and higher order temperature dependencies remain. In particular, a plot with temperature on the x-axis and output voltage on the y-axis results in an approximately parabolic curve that reaches a maximum at about the ambient temperature of the bandgap reference.
Some conventional bandgap references even substantially reduce much of the second and higher order temperature variations in the output voltage. One such bandgap voltage reference circuit is disclosed in U.S. Pat. No. 5,767,664 (hereinafter referred to as the ""664 patent), which issued Jun. 16, 1998 to B. L. Price. FIG. 3 illustrates such a bandgap reference 300.
The bandgap reference 300 includes the conventional bandgap reference 200 of FIG. 2, but also includes a V-to-I converter circuit 304 with two differential pair segments 306 made up of MOSFETs M1-M4. A current mirror 308 is formed with MOSFETs M5 and M6 so as to extract a correction current, ICORR, from the VB node. The correction current reduces a significant portion of the remaining temperature dependencies that were present in the bandgap reference 200. Accordingly, the voltage at node VB is relatively temperature stable. As a consequence, the output voltage of the bandgap reference 300 is a DC voltage that is relatively stable with temperature changes as compared to the prior bandgap reference 200.
In order for the correction current to reduce temperature errors, the differential pairs 306 are tuned to provide an appropriate current component at given temperatures. One current source 308 is provided for each differential pair 306. A PTAT voltage is applied to the gate terminal of the left MOSFET in each differential pair (e.g., M1 for differential pair 306xe2x80x2, and M3 for differential pair 306xe2x80x3). A substantially constant voltage is tapped onto the gate terminal of the right MOSFET in each differential pair (e.g., M2 for differential pair 306xe2x80x2, and M4 for differential pair 306xe2x80x3). As the temperature varies the voltage applied to the gate of the left MOSFET in each differential pair will change. Note that the relatively constant voltage applied to the gate of MOSFET M2 will be lower that the relatively constant voltage applied at the gate of MOSFET M4 due to the voltage division provided by resistors R4A, R4B and R4C.
Each of the differential pairs 306 generates a component of the correction current. For example, consider the differential pair 306xe2x80x2 which contributes a component of the correction current. At very low temperatures, the gate voltage of MOSFET M1 is lower than the gate voltage at M2. Accordingly, most of the current I1 is diverted through M1 to contribute to ICORR via current mirror 308. However, the MOSFET M4 is substantially off. Accordingly, at lower temperatures, the corrective current is approximately proportional to current I1.
As the temperature rises, the gate voltage of M1 becomes the same as the gate voltage of M2. Accordingly, only half of the current I1 would pass through M1 to contribute to curvature correction current ICORR. This temperature is often referred to as the xe2x80x9ccrossing pointxe2x80x9d. At very high temperatures, the gate voltage of M1 is higher than the gate voltage of M2. Accordingly, very little of the current I1 passes through M1 to contribute to the error current.
Accordingly, by adjusting the crossing point of each differential pair, one may change the current contribution profile of each differential pair until the sum of the contributions results in a correction current that generally reduces the temperature error in the output voltage. In FIG. 3, the crossing points are set by fine tuning the size of the resistors R4A, R4B, and R4C.
The bandgap reference 300 provides a significant improvement in the art. However, there is still some degree of temperature dependency in the output voltage, despite the correction current. Accordingly, what are desired are bandgap circuits and methods for more precisely generating a correction current so that temperature dependencies in the generated output current may be even further reduced.
The foregoing problems in the prior state of the art have been successfully overcome by the present invention, which is directed to bandgap reference circuits and methods that generate a correction current by using differential pairs using positive as well as negative temperature drift voltage sources to perform current steering or diversion in each differential pair.
In accordance with the present invention, a bandgap voltage reference circuit includes a bandgap voltage source that is configured to generate a bandgap voltage during operation, the bandgap voltage having strong temperature dependencies. For example, one bandgap voltage reference source may be a bipolar transistor having a forward-biased base-emitter junction. In that case, the voltage across the base-emitter region (VBE) would be a bandgap voltage having heavy temperature dependencies. Such temperature dependencies include first, second, and higher order temperature dependencies. A Proportional-To-Absolute-Temperature (PTAT) voltage source may add a PTAT voltage to the bandgap voltage so as to substantially reduce the first-order temperature dependencies. However, even in that case, second and higher order temperature dependencies would still remain.
The bandgap voltage reference circuit also includes one or more differential pairs. Each differential pair comprises a current source, a voltage source that generates a voltage that has a negative temperature shift (i.e., the voltage reduces as temperature rises), as well as a voltage source that generates a voltage that has a positive temperature shift (i.e., the voltage rises as temperature rises). One of the MOSFETS of the differential pair has its gate terminal coupled to the positive temperature shift voltage, while the other MOSFET has its gate terminal coupled to the negative temperature shift voltage. Accordingly, the principles of the present invention use a positive and negative temperature shift voltage to control current diversion in the differential pairs. This contrasts with the conventional bandgap references that use only the positive temperature shift voltage to control current diversion in differential pairs.
Using both positive and negative temperature shift voltages to control current diversion results in significant advantages. In particular, as temperature rises, not only does one MOSFET turn on, but the other MOSFET also turns off. This results in faster convergence from a total contribution state in which a MOSFET is turned on completely allowing all of the current from the current source to contribute to the correction current, to a zero contribution state in which the MOSFET is turned off completely allowing none of the current from the current source to contribute to the correction current. This allows for better resolution in designing a correction current. Accordingly, more accurate correction currents may be generated to make a more temperature stable output voltage.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.